Variable reactance



Sept. 12, 1950 M. G. cRosBY I 2,521,694

' VARIABLE REACTANCE Filed Nov. 7, 1946 3 Sheets-Sheet 1 P 1950 M. G. CROSBY 2,521,694

VARIABLE REACTANCE Filed Nov. '7, 1946 3 Sheets-Sheet 2 p 1950 M. G. CROSBY 2,521,694

VARIABLE REACTANCE Filed Nov. 7, 1946 3 Sheets-Sheet 3 Patented Sept. 12, 1950 VARIABLE REACTANCE Murray G. Crosby, Upper Montclair, N. J., as-

signor to Boonton Radio Corporation, Boonton, N. J., a corporation of New Jersey Application November 7, 1946, Serial No; 708,408

4 Claims. (Cl. 332-19) 1 This invention relates to variable reactances of the vacuum tube type in'which a circuit of the vacuum tube has the characteristics of a reactance which may be varied in effective value by altering the potential applied to an element of the tube.

The reactiv effect of the prior reactance tubes has been such that, when coupled with tuned circuits to produce phase or frequency modulation, the degree of modulation produced by a given' element voltage variation changed with the resonant frequency of the tuned circuit; This variation with frequency of the frequency or phase deviation is inherent in the prior reactance tubes which, for any given reactance value, produce a phase or frequency deviation which either increases or decreases as the resonant frequency of the tuned circuit is changed. The tuned circuit may be coupled with an oscillator tube for generation of frequency modulation, or with an amplifier tube for the generation of phase modulation.

The circuits contemplated by this invention have as their object the generation of a constant amount of phase or frequency deviation over a range of frequencies of the tuned circuit for a given value of applied control potential. Such a deviation characteristic is desirable for various applications, one of them being in measuring and testing equipment, such as, for example, a frequency modulated signal generator.

An object of the present invention is to provide a variable reactance which, when placed across a tunable oscillator or transmission circuit, will effect a constant frequency or constant phas deviation over the tunable range of the oscillator or transmission circuit. An object is to provide reactance tubes which exhibit a constant deviation characteristic over a tuning range and in which the frequency or phase deviation developed by an applied control potential of a given value is maintained constant by electrical circuit elements of either fixed or variable magnitudes. Other objects are to provide reactance tubes in which the action of phase shifters of more or less conventional type is modified to develop a reactive effect which changes with frequency at such rate as to afford a constant deviation over a range of frequencies. Other objects are to provide reactance tubes having networks which effect a substantially constant phase shift and an attenuation which varies with frequency in such, manner as to develop fre-' quency or phase deviations which are substan- 2 tially independent of the nominal frequency of the oscillator or amplifier circuits.

For convenience of description, reference will be made particularly to the frequency modulation type of circuits but the application of the invention to phase modulation is obvious without specific reference thereto.

In reactance-tube frequency modulation there are two general types of reactance-tube circuits employed. One type has the characteristics of a controllable inductance and the other has the characteristics of a controllable capacitance. The magnitude of the equivalent inductance or capacitance produced by the conventional reactance-tube circuit with a two-element phase shifter is independent of frequency. The reactance-tube circuit is placed in parallel with the tuned circuit to be modulated so that the inductive type may be considered as adding a controllable inductance in parallel with that of the tuned circuit while the capacitive typ adds a controllable capacitance in parallel with that of the tuned-circuit.

For the inductive type of reactance tube, the resulting total tuned circuit inductance, Lt, is that of the tuned circuit and the reactance tube in parallel or,

where L is the inductance of the tuned circuit and Le is the effective inductance of the reactancem ter ice) in which C is the capacitance of the tuned circuit and 21 /276 The quantity is normally small in comparison with unity, so that .2) simplifies to The first term fiof this equation is the resonantfrequency of the tuned circuit alone and the second term is the frequency deviation which resuits from the action of the reactance-tube circuit. The frequency deviation is seen to be proportional to the resonant frequency fr, and the ratio L/Iae, between th tuned-circuit inductance and the effective inductance of the reactance tube.

For the capacitive type of reactance tube, the resulting total tuned-circuit capacitance is the capacitance C of the tuned circuit and the equivaient capacitance, Co, of the reactance tube in parallel. The frequency is then given by 1 C. 14 Tv'LT1 TY fi 4) For the practical case,

Cc/C

is small compared with unity, so that,

f=f.(1 %)=f.-f;

It is thus seen that the frequency deviation pro- L frg In this type of circuit, L is fixed and Le is independent of frequency. The result is a fixed percentage variation of inductance which produces a fixed percentage frequency variation. The frequency deviation in cycles is therefore proportional to the resonant frequency.

If the tuned circuit is of the variable-capacitance, fixed-inductance type, and a capacitive reactance tube is used, the percentage capacitance variation changes with frequency as the tuning capacitance is varied. That is, the ratio Ce/C varies with frequency since C is varied. The capacitance C is varied in accordance with the formula which may be rewritten,

l L(21rf,) 4 (6) Substituting Equation 6 in (5) gives,

f==fr21r CeLfr (7) In the frequency deviation given by the second term in (7), the quantities Ca and L are independent of' frequency fOr the fixed-inductance type of tuned circuit. The percentage frequency deviation is thus proportional to the square of the resonant frequency. The deviation in cycles is proportional to the third power of the resonant frequency.

If the tuned circuit is of the variable-inductanoe fixed-capacitance type, and the reactance tube is inductive, the ratio L/Lc varies with fre- 4 quency. The tuned circuit inductance L, is tuned in accordance with 2m which may be rearranged to.

Substituting (8) in (3) gives La and C are independent of frequency for this combination so that the frequency deviation is inversely proportional to the resonant frequency. The percentage frequency deviation is therefore inversely proportional to the square of the frequency.

If the tuned circuit is of the fixed-capacitance. variable-inductance type and a capacitive type variable reactance tube is employed, the frequency deviation may be taken from Equation 5, The frequency deviation is and, since Ce and C are both constant for this combination, the frequency deviation is directly proportional to fr and the percentage deviation is independent of frequency. The following table summarizes the frequency deviations obtained with the various types of tuned circuits and reactance tubes employing the conventional twoelement phase-shifting circuits.

It may be seen that when the fixed element of the tuned circuit and the reactance tube type are the same (both capacitive or both inductive), the deviation sensitivity is proportional to the operational frequency. For opposite reactance combinations, the deviation is either inversely proportional to the frequency or directly proportional to the third power of the frequency.

The plate circuit of a reactance tube appears as a reactance by virtue of the reactive current it draws due to the phase-shifted voltage fed from the plate to the grid. The impedance Zc, looking into the plate circuit is given by In! Z,=E. (l0) in which Ed" is the plate alternating voltage and i the plate current.

If the current drawn by the phase-shifter is assumed to be negligible in comparison with the plate current of the reactance tube, the current ip iS,

ip=enGm (11) in which Gm is the transconductance of the tube. The grid voltage e; is

in which B is the transmission of the phaseshifter (output voltage/input voltage), and c is the phase shift produced by .the phase-shifter. Substituting 12) in (11) and the resulting plate current in gives,

Eeilf e Thus, the impedance looking into the plate circuit of the reactance tube is composed of a series combination of a resistance Re and a reactance Xe. If o is between zero and +180-degrees (leading), the equivalent reactance is negative and therefore represents a capacitance. Likewise, if 6 is between zero and -'180 degrees (lagging), the equivalent reactance is positive and therefore represents an inductance. For the practical case in which it is close to :90 degrees, the resistive component may be neglected and the reactance becomes,

In the case of the inductive type of reactance tube, the equivalent inductance is,

when the equivalent inductance of Equation 18 is inserted in (3), the frequency is,

in (21) shows that the deviation is independent of frequency if the phase-shifter transmission B, and the tuned circuit capacitance C, are also independent of frequency. Hence, constant-deviation operation may be obtained by the use of a fixed-capacitance, variable inductance tuned circuit and a capacitive type reactance tube employing a phase-shifter having a transmission independent of frequency.

when an inductive reactance tube is used with wX, w Inserting the c. from (20) in (5) gives f f 2 f m The deviation term,

a fixed-capacitance, variable-inductance tuned in circuit, the frequency may be found by inserting Equation (18) in Equation 9 to give J=M5 (22) The deviation term, as. 41C

in (22) shows that the deviation is independent of frequency if the transmission B is also independent of frequency. Hence, constant-deviation operation may be obtained by the use of a phase shifter having a transmission independent of frequency.

When a capacitive reactance tube is used with a variable-capacitance, fixed-inductance tuned circuit, the frequency may be found by inserting Equation 20 in (7) to give,

The deviation term in (23) is proportional to the square of the resonant frequency. Hence, if the phase shifter transmission is made inversely proportional to the square of the resonant frequency, constant-deviation operation will be obtained.

The particular phase-shifter transmission for producing constant-deviation operation with the various combinations of tuned circuits and reactance tubes are listed in the following table.

Table II Phase-shifter gm gleggtiance FlflIlSInifiiOl ype or cons n element deviation L l L Bo: f.i2 L C Ba' fi-Z C L B Constant C C B Constant It can be seen that when the fixed tuning element is the inductance, the transmission should be inversely proportional to the square of the frequency, whereas, if the fixed tuning element is the capacitance, the transmission should be constant with frequency.

when the tuning of the tuned circuit to be reactance modulated is accomplished by a variation of both the inductance and capacitance, such as by means of a butterfly circuit or a permeabilitytuned inductance ganged with the variable con- "denser, constant-deviation operation may be obtained by controlling the rate of change of inductance and capacitance with frequency. In

the case of the inductive reactance tube, the resultant frequency is given by Equation 3 which shows that the deviation is directly proportional to the resonant frequency.

If the inductance, L. is varied with the frequency v in a manner such that,

where k is a constant, the deviation term becomes which is constant with frequency. The capacitance change required with an inductance change 7 given by Equation 24 maybe found by substituting (24) in the relation for the resonant frequency to give,

where k1 is a constant. Dividing (24) by (26) gives the L/C ratio of the tuned circuit which is where k: is a constant. Equation 2'? shows that constant-deviation operation may be obtained by arranging the variable-industance and variablecapacitance to be tuned in a cooperative manner such that the ratio of the inductance to the capacitance is constant. This design requires that the inductance and the capacitance each vary in inverse proportion to the resonant frequency.

When a capacitive reactance tube is used with a variable inductance, variable capacitance tuned circuit, the conditions required for constant-deviation operation may be determined from a study of Equation 5. The deviation term in that equation is directly proportional to the resonant frequency and inversely proportional to the tuned circuit capacitance. Hence, if the tuned circuit capacitance is varied so that,

where K is a constant, the deviation term becomes Q 2K which is independent of frequency. Substituting (28) in the relation for the resonant frequency, and rearranging in the manner of Equations 25 and 26 gives,

is inversely proportional to the fourth power of the frequency. This calls for a capacitance variation directly proportional to the resonant frequency and an inductance variation inversely proportional to the third power of the frequency.

The invention is not limited to the described electrical methods as various mechanical arrangements of ganged adjustable impedances may be employed to obtain a frequency deviation which is determined by the impressed control voltage and is independent of the frequency of the tuned oscillator circuit. The tuning dial may be mechanically coupled to a potentiometer through which an energizing potential is applied to an element of the reactance tube, the arrangement being such that the effective mutual conductance of the reactance tube is altered with frequency to compensate for the variation in deviation which normally results from the use of a simple two-element phase shifter. Alternatively,

the mechanical coupling may be applied to an element of the phase-shifter network to alter, with frequency, either the transmission or the phase shift, or both the transmission and the phase shift, to obtain the desired constant deviation.

1 The several described methods or circuit arrangements for employing reactance tube.circuits to obtain constant-deviation frequency or phase modulation of a tunable oscillator or amplifier circuit may be summariied as follows:

Constant-deviation methods 1. Control of mutual conductance of reactance tube by a potentiometer ganged to the tuning dial.

2. Control of the transmission or phase shift of the reactance tube system by ganging a phaseshifter element to the tuning dial.

3. Fixed-inductance tuned circuit with inductive reactance tube having a phase-shifter transmission inversely proportional to the square of the frequency.

4. Fixed-inductance tuned circuit with capacitive reactance tube having a phase-shifter transmission inversely proportional to the square of the frequency.

5. Fixed-capacitance tuned circuit with inductive reactance tube having a phase-shifter transmission independent of frequency.

6. Fixed-capacitance tuned circuit with capacitive reactance tube having a phase-shifter transmission independent of frequency.

7. Variable-inductance, variable-capacitance tuned circuit with constant L/C ratio, and an inductive reactance tube.

8. Variable-inductance, variable-capacitance tuned circuit with L/C ratio inversely proportional to the fourth power of the frequency, and a capacitive reactance tube.

9. Combinations of two or more of the above Methods 1 to 8.

The objects of the invention as stated above may be attained according to the described methods with circuit and apparatus arrangements as shown in the accompanying drawings in which:

Fig. l'is a diagram of a reactance tube circuit embodying the invention and operating in accordance with the above-defined Method 1;

Fig. 2 is a schematic circuit diagram of reactance tube systems operating in accordance with Method 2;

Figs. 3 and 4 are circuit diagrams of reactance tube systems according to Method 3;

Fig. 5 is a curve sheet showing the attenuation-frequency characteristic of the phase shifter circuits of Figs. 3 and 4;

Fig. 6 is a circuit diagram of a reactance tube system according to Method 4;

Fig. 7 is a diagram of a reactance tube system operating according to methods 5 and 6;

Fig. 8 is a curve sheet showing the attenuation-frequency characteristics of the circuits of Figs. 7, 9 and 10;

Fig. 9 is a diagram of a reactance tube operating according to Method 6;

Fig. 10 is a diagram of reactance tube systems operating according to Methods 5 and 6;

Fig. 11 is a circuit diagram of a reactance tube system associated with a butterfly or equivalent circuit operating according to Methods 7 and 8; and

Fig. 12 is a circuit diagram of a reactance tube system operating in accordance with Method 9, i. e. according to a combination of Methods 2 and 3.

In Fig. 1 of the drawings. the reference numeral I identifies the fixed inductance of an oscillator tank circuit which is tuned by a condenser 2. A reactance tube 3 is connected across the tuned circuit, and a phase shlfting network comprising serially connected impedances Z, Z and a blocking condenser 4 is connected between the anode A and cathode K of the tube. The Junction of the impedances, and also a lead 5 from a source of modulating voltage, are connected to the control grid G1, and the energizing voltage is applied to the screen grid G2 through the contact arm of a potentiometer 5. contact arm is mechanically coupled to the tuning condenser 2 by a linkage or gearing which is indicated schematically by the broken line I, and constant deviation over the frequency range is obtained according to Method 1. Conventional resistance and by-pass condenser elements are illustrated but will not be specifically described and identified by reference numerals.

Inspection of Table I shows that the deviation developed by the Figure 1 circuit will not be constant if the mutual conductance. of the reactance tube remains constant. It is seen that for two cases the deviation is directly proportional to the frequency, and for the other two the deviation is either directly proportional to the cube of the resonant frequency or inversely proportional to the first power of the resonant frequency. Hence for the first two cases, the screen-grid potential must be reduced as the frequency is increased so that the mutual conductance will be made inversely proportional to the resonant frequency. For the third case, the mutual conductance must be varied so as to be inversely proportional to the cube of the resonant frequency. For the fourth case, the mutual conductance must be increased as the frequency is increased so as to compensate for the inverse proportionality of the deviation. The increase will be such as to make the mutual conductance directly proportional to the resonant frequency. A circuit operating according to Method 2 is shown schematically in Fig. 2. The energizing potentials applied to the elements of the reactance tube 3 are not varied but one impedance, for example the impedance Z of the phaseshifter network Z, Z, is coupled by a mechanical'link 'I to the tuning element X of the tank circuit which comprises the adjustable impedance X and a fixed impedance X. The operational characteristics of the mechanical link will of course depend upon the nature of the several elements Z, Z and X, X of the phase shifter and of the tuned circuit. These impedances may be selected for operation according to any one of 'the combinations of Table II, and the required change of the impedance Z with frequency is listed in the tabulation. For example, for a permeability-tuned circuit, the mechincal coupling to the phase-shifter impedance Z must maintain a constant phase-shifter transmission whether the reactance tube 3 has an inductive or a capacitive characteristic.

The circuits of Figs. 3 and 4 each include a capacitively tuned circuit I, 2 and an inductive reactance tube 3 with a phase-shifter 8 which provides a constant deviation by operation according to Method 3. In the Fig. 3 circuit, the phase-shifter 8 takes the form of a resistance 9 in series with a series-tuned circuit comprising an adjustable condenser Ill and an inductance I I. The junction of resistance 9 and condenser I0 is connected to the control grid of the reactance The ser 4. The modulating potential may be applied to grid at the low side of the grid resistor I2, as indicated by the arrow on the lead to the resistor I2. The condenser I0 and inductance II of the phase-shifter are selected and adjusted ,to resonate at a frequency F which is higher than the tuning range F1 to F2, of the oscillator circuit I 2, thereby imparting to the phase-shifter a transmission characteristic as shown by curve I of Fig. 5. The transmission varies inversely with the square of the frequency over the operating range of from frequency F1 to frequency F2.

The circuit of Fig. 4 provides a constant deviation over a somewhat longer operating range. The phase-shifter network comprises resistors I3, I4 serially connected between the blocking condenser 4 and the control grid of the reactance tube 3, capacities I5, I6 connected between the grid ends of resistors I3, I4 and ground, a capacity I1 shunting the serially connected resistors. The capacities I6 and I! may be the inherent grid-cathode and grid-anode capacities of the tube3, or either or both may include physical condensers in addition to the interelectrode capacities. The capacity I5 is preferably adjustable, as is indicated diagrammatically, as the maximum and minimum carrier frequencies F1, F2 between which constant deviation is obtained may be adjusted over a somewhat limited range by adjusting the effective value of capacity I5. The bridged-T network is tuned with its rejection point at frequency F, see Fig. 5, and the phase-shift is of approximately and a transmission which is inversely proportional to the square of the frequency.

The reactance tube 3 is of the inductive type in the circuits of Figs. 3 and 4, but inspection of Table II indicates that the same attenuation characteristic will afford constant deviation when a reactance tube of the capacitive type is employed with a variable-capacitance tuned circuit. The circuits of Figs. 3 and 4 may be converted to operate as capacitive type reactance tubes, and with the same transmission characteristic, by introducing an amplifier tube between the phase-shifter 8 and the reactance tube 3 to reverse the polarity of the voltage fed from the phase-shifting network to the reactance tube. This conversion of the Fig. 3 circuit for operation according to Method 4 is illustrated in Fig. 6. The phase-shifted voltage is impressed upon the control grid of an amplifier tube I8, and the tube I8 is resistance-coupled to the reactance tube 3 through a coupling condenser I9 and resistances '20, 20 in the anode circuit of tube I8 and in control grid circuit of tube 3, respectively. A "peaking inductance 2| is preferably included in the anode circuit to provide a peaking circuit such as used in wide-band television resistance-coupled amplifiers.

The oscillator circuits of Figs. 7, 9 and 10 are of the inductively tuned type, and the phaseshifters of the reactance tubes have a constant transmission over the tuning range for constant- 11 shifter 24 may impart an inductive character to the reactance tube for operation according to Method or capacitive character for operation according to Method 6. A limiter 25 is interposed between the phase-shifter 24 and the control grid of reactance tube 3 to convert the frequencyvariant output characteristic of the phase-shifter to a fiat overall output characteristic. The output of the phase-shifter 24 rises with frequency, "as shown by curve II of Fig. 8, when the phaseshifter develops a leading voltage, and decreases with frequency according to curve 111 when the phase-shifter develops a lagging voltage. The reactance tube should develop a constant output, for a given oscillator output voltage, and the limiter 25 must be designed to provide a fiat overall transmission characteristic, see curve IV of Fig. 8, for the phase-shifter and limiter combination. The oscillator should have an output which is substantially independent of the tuning of the oscillator circuit I, 2'. Any known or desired arrangement may be employed and, for purpose of illustration, the Fig. 7 circuit includes a constant output oscillator of the type described in my prior Patent No. 2,269,417.

A specific embodiment of the Figure '7 type of circuit is illustrated in Figure 9. The phaseshifter comprises a condenser 24C in series with a resistor 24R, and the reactance of the condenser is relatively large with respect to the resistance 24R. over the tunable range of the circuit to be modulated. The double triode 26 is a limiter of the type described in my prior Patent No. 2,276,565; the grid of one triode section being connected to the junction of condenser 24C and resistor 24R. The limited output of the phase-shifter appears in the plate circuit of the other triode section and is fed to the control grid of the reactance tube 3. Modulation of the reactive eifect to produce frequency deviation may be accomplished by applying a modulating potential to an element of the tube 3 or, as illustrated, by impressing the modulating potential on the grid of the output triode section of tube 26. The source of modulating potential is connected through a jack 2'! to the primary winding of a transformer 28, and the secondary winding is connected between ground and the control grid of the output triode section of tube 26.

The desired constant transmission of the phase-shifted voltage may ,also be obtained by Methods 5 and 6 with a circuit of the general type illustrated in Fig. 10. An automatic volume control system, comprising an amplifier 29 and rectifier 30, is connected between .the phase-shifter 24 and the control grid of the reactance tube 3. The amplitude of the phaseshifted voltage is held constant by the variation in amplifier gain which is effected in known manner by applying the rectified output of rectifier 30 as a control potential on the control grid or grids of the amplifier 29. As in the previously described embodiment, the tunable circuit of the oscillator or amplifier is of the fixed-capacity and variable-inductance type.

The apparatus illustrated in Fig. 11 employs Method '7 or 8 to obtain constant deviation. A tuned circuit comprising a variable inductance I and a variable capacitance 2 is connected across an inductive or a capacitance type of reactance tube 3 which has a simple two-element phase-shifter 24. The tuned circuit may be of the butterfiy" type described by Eduard Karplus in Proceedings of the Institute of Radio Engineers, July 1945, vol. 33, page 426, or it may be one of the other possible arrangements such as a variable capacitance 2 coupled by mechanical link 'I to the core of a permeability-tuned inductance I'. If the reactance tube is the simple two element capacitive type. Equations 28 and 29 above show that the capacitance should be varied proportional to the resonant frequency, and the inductance varied inversely proportional to the cube of the resonant frequency. When the reactance tube is of the inductive type, Equations 24 and 26 show that both inductance and capacitance should be made inversely proportional to the resonant frequency. These requirements as to the rule of variation of the inductance and capacity may be met by the design of the type of butterfly circuit, or by proper combination of variable-capacitance and variableinductance characteristics.

The described circuits for effecting constant deviation by one of Methods 1 to 8 inclusive may of course be combined in various ways for operation according to method 9. Such combinations provide over all operating characteristics which cannot be obtained as readily or as economically with a circuit which operates according to only one of the specific methods of obtaining constant-deviation modulation. vantage or advantages of the combination of methods may be improved sensitivity, a wider tuning range, greater accuracy within a given tuning range, or greater latitude in production tolerance.

A typical combination circuit, as illustrated in Fig. 12, functions according to a combination of Methods 2 and 3 to extend the tuning range over which the deviation can be maintained constant, within a particular limit of percentage error, by either one of the methods. The Fig. 12 circuit includes all of the elements of the Fig. 4 circuit, and the several elements are identified by the reference numerals of Fig. 4, but will not be specifically described. The circuits differ in that the phase-shifting condenser I5 of Fig. 12 is coupled mechanically, as indicated by the broken line 1', to the tuning condenser 2 of the oscillator circuit. Adjustment of the condenser l5 alters the upper and lower limits of the frequency range over which the Fig. 4 circuit maintains the modulation deviation constant to a preselected accuracy, and the effective operating range is therefore extended by ganging the phase-shifting condenser IE to the tuning control. The mechanical coupling may be of any known or desired type to effect a simultaneous adjustment, in opposite sense, of the circuit-tuning condenser 2 and the phase-shifting condenser l5.

This application is a continuation-in-part of my prior application Ser. No. 642,763, filed January 22, 1946.

The invention is not limited to the particular circuits herein shown and described as various modifications which may occur to those skilled in the art fall within the spirit and scope of the invention as set forth in the following claims.

I claim:

1. In a frequency or phase modulating system, a radio frequency circuit including a capacitive and an inductive impedance, tuning means for adjusting said capacitance to tune said circuit over a frequency range, a reactance tube having an anode and a grid cooperating with a cathode, said anode and cathode being effectively connected across said radio frequency circuit, a phase-shifting circuit for feeding a phase The admemes shifted voltage from the anode to the grid of said reactance tube. and means for applying a mod.- ulating voltage to the radio frequency voltage developed across said radio frequency circuit; said phase-shifting circuit being a bridged-T network of resistive and capacitive impedances.

2. In a frequency or phase modulated systom, the invention as recited in claim 1, wherein a capacitive impedance of said phase-shifting network is adjustable; in combination with means angingsaid adjustable capacitive im- Pedance of the network to said tuning means of said radio frequency circuit for adjustment simultaneously with and in opposite sens to adjustments of the capacitive impedance of the radio frequency circuit.

8. In a frequency or phase modulated system, the invention as recited in claim 1, wherein said network impedances have relative values which tune said bridged-r network to a minimum on at a frequency above the timing range of said radio frequency circuit.

the grid, thereby to modulate MURRAY G. CROSBY.

REFERENCES CITEE The following references are of record in th file of this patent:

UNITED STATES PATENTS Number Name Date Re.22,834 Alvira Jan. .28, 1947 '..2,382,436 Marble Aug. 14, 1945 

